As shown in FIG. 1, conventionally an electric generator 10 comprises a rotor 12 carrying axial field or rotor windings 13 producing a magnetic flux field that rotates within a stationary armature or stator 14. One end 15 of the rotor 12 is drivingly coupled to a steam or gas driven turbine (not shown in FIG. 1) for providing rotational energy to turn the rotor 12. The opposing end 16 is coupled to an exciter (not shown) for providing the direct current carried by the rotor windings 13.
The stator 14 comprises a core 17 including a plurality of thin, high-permeability circumferentially slotted laminations placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils 18 (see FIG. 3) are wound in the inwardly directed slots of the stator core 17. Alternating current is generated in the stator windings 18 by the action of the rotating magnetic filed emanating from the rotor windings 13. The current is carried to the main leads 19 for connection to an external electrical load.
The rotor 12 and the stator 14 are enclosed within a frame 20. Each rotor end comprises a bearing journal for mating with bearings 30 attached to the frame 20. The rotor 12 further carries a blower 32 for forcing cooling fluid through the generator elements as described further below. The cooling fluid is retained within the generator 10 by seals 34 located where the rotor ends penetrate the frame 20. The generator 10 further comprises coolers 36 through which the cooling fluid passes to release the heat absorbed from the generator components, after which the cooling fluid is recirculated through the generator elements.
FIG. 2 further illustrates the elements of the rotor 12, including ventilation slots 50 on opposing ends of the rotor 12 and ventilation ports 52 located near the rotor center. As will be described further below, cooling fluid passes through the ventilation slots 50 and the ventilation ports 52 for cooling the rotor 12. The direct current generated by the exciter is coupled to the rotor windings through axial leads 54. The coil ends are held in place by retaining rings 56.
FIG. 3 is a cross-sectional view of the stator 14, illustrating the various components described herein, including a face 60 of one lamination of the stator core 17 and the inwardly directed slots 62 carrying the stator coils 18. A somewhat distorted perspective view of the center region of stator 14 is provided to illustrate stator coils 18 and slots 62 extending along the axial length of stator 14.
Generator cooling is required to remove the heat energy produced by electrical losses resulting from the large currents flowing through the generator conductors, including the direct current flowing through the rotor windings 13, and the alternating current induced in the stator coils 18. Mechanical losses such as windage caused by the spinning rotor 12 and friction at the bearings 30 are also heat sources.
As generator electrical output ratings increase, additional heat is generated within the generator and must be removed through the use of more effective cooling systems. Generally, as the heat removal requirements increase, the basic premise of the cooling system operation progresses from air cooling, to hydrogen cooling, to hydrogen inner cooling, and finally, to cooling the stator with flowing water. Certain of these cooling techniques can also be used in combination, and there are multiple variations for each cooling system. Each cooling system type is described briefly below.
Air-cooled generators can be configured as either open or closed. Open air-cooled generators use outside air. The air passes through the generator components only once, after which it is exhausted back outside the generator. Closed air-cooled cooling systems include a heat exchanger, also referred to as a cooler, for cooling the heated airflow after it has passed through the generator. The cooled air is then recirculated back through the generator. Cold water is pumped through tubes of the heat exchanger over which the hot air passes, transferring heat from the air to the water.
Although air can be used as the cooling fluid, hydrogen is preferred as it possess excellent thermodynamic and heat transport properties, is lighter than air, and is 10 to 20 times more efficient as a cooling medium than air. One important negative aspect of hydrogen cooling is the explosive mixture formed by hydrogen and air over a wide range of hydrogen concentrations. Therefore, the seals 34 are provided at the boundaries of the generator frame 20 to prevent hydrogen leakage.
Hydrogen cooled generators are subdivided into two groups, conventional-cooled and inner-cooled. In the conventional system, the hydrogen flow removes excess heat energy from the rotor 12 and stator 14 by circulating hydrogen around and through coolant paths within and proximate the generator components, including especially the rotor 12 and the stator 14. The blower 32 creates high and low pressure zones within the generator, establishing hydrogen gas flow paths from high-pressure zones to low-pressure zones to remove heat from the generator components.
To provide rotor cooling, the hydrogen is directed through channels (not shown) in the hollow rotor windings, entering at the ventilation slots 50 at both rotor ends and exiting into a rotor/stator gap 64 (see FIG. 1) via the ventilation vents 52 in a mid-region of the rotor 12. The flow continues in the gap 64 as the hydrogen flows back to hydrogen coolers 36.
Cooling hydrogen is also directed through the stator core 16 in both the radial and the axial directions. In radially cooled generators, the blower 32 causes hydrogen to flow axially along the outside of the stator 14 then radially inwardly toward the gap 64 (due to the lower pressure within the gap 64) through openings or vents between certain of the stator core laminations. The hydrogen in the gap 64 is directed back to the coolers 36 where the absorbed heat is removed, and the hydrogen is then recirculated back through the generator 10. In an axially cooled stator, the hydrogen flow is directed from the coolers 36 axially along the outside of the stator 14 to the opposite end of the stator 14 then through axial ducts in the stator core 16 back toward the coolers 36.
As mentioned, the heated hydrogen flow exiting from the stator 14 and rotor 12 is directed by the blower 32 through the hydrogen coolers 36 mounted at the turbine end of the generator frame 20. Within the coolers 36 the hydrogen is cooled as it passes over water filled tubes. The cooler hydrogen flow is recirculated to continue the heat removal process.
A hydrogen inner-cooled system includes cooling ducts in the form of hollow passages in the stator coils 18, in addition to the axial or radial stator cooling ducts in the stator core 16 as described above. As the hydrogen passes through the cooling ducts, heat is absorbed from the conductors of the stator coils 18.
In a water cooling system the rotor 12 and stator core 16 are cooled with hydrogen as described above, while the stator coils 18 are cooled by pumping water through hollow conductors forming the stator coils 18. The water is cooled by outboard heat exchangers and recirculates through the stator coils 18.